Cryogenic refrigeration apparatus with automatic temperature control and automatic gas balance control

ABSTRACT

Apparatus and method for refrigerating articles comprising a heat insulated tunnel, a conveyor therein for passing articles to be refrigerated through the tunnel, introducing means for a cryogen into the chamber that comprises a cryogen gas therein, circulating means for circulating the gas in a circulation loop including a first reach in direct heat exchange contact with the articles and a second reach out of this direct contact, a gas exhaust for expelling refrigerant gas from the tunnel spaced from the loop and adjacent the tunnel entrance, a flow restrictor between the circulation loop and the exhaust for restricting volumetric flow of gas from the circulation loop to the exhaust and thereby maintaining a rapid gas flow rate in the circulation loop, a gas outlet vent located adjacent to the tunnel exit, temperature control for controlling the rate of introducing the cryogen as a function of the gas temperature in the loop and preferably but not necessarily in the second reach in the loop and gas balance control means for proportioning gas flow between the loop and the vent as a function of the gas temperature at the vent.

United'States Patent Wagner [451 Aug. 12, 1975 Primary ExaminerWilliamF. ODea Assistant ExaminerRonald C. Capossela Attorney, Agent, orFirm-1-1ofgren, Wegner, Allen, Stellman & McCord [57] ABSTRACT Apparatusand method for refrigerating articles comprising a heat insulatedtunnel, a conveyor therein for passing articles to be refrigeratedthrough the tunnel, introducing means for a cryogen into the chamberthat comprises a cryogen gas therein, circulating means for circulatingthe gas in a circulation loop including a first reach in direct heatexchange contact with the articles and a second reach out of this directcontact, a gas exhaust for expelling refrigerant gas from the tunnelspaced from the loop and adjacent the tunnel entrance, a flow restrictorbetween the circulation loop and the exhaust for restricting volumetricflow of gas from the circulation loop to the exhaust and therebymaintaining a rapid gas flow rate in the circulation loop, a gas outletvent located adjacent to the tunnel exit, temperature control forcontrolling the rate of introducing the cryogen as a function of the gastemperature in the loop and preferably but not necessarily in the secondreach in the loop and gas balance control means for proportioning gasflow between the loop and the vent as a function of the gas temperatureat the vent.

16 Claims, 11 Drawing Figures GAS BALANCE CONTROL INSTRUMENT TEMPE RATURE CONTROL INS TRUME N T [75] lnventor: Richard C. Wagner, Darien,111.

[73] Assignee: Hollymatic Corporation, Park Forest, 111.

[22] Filed: Dec. 14, 1973 [21] App]. No.: 424,768

Related US. Application Data [63] Continuation-impart of Ser. Nos.264,133, June 19, 1972, Pat. No. 3,824,806, and Ser. No. 390,755, Aug.23, 1973, Pat. No. 3,871,186.

[52] US. Cl. 62/208; 62/216; 62/374; 62/380 [51] Int. Cl. F25b 41/06[58] Field of Search 62/63, 208, 209, 216, 374, 62/375, 380

[56] References Cited UNlTED STATES PATENTS 3,345,828 10/1967 Klee et a162/63 3,553,973 l/1971 Moran 62/63 3,605,434 9/1971 Boese.... 62/3743,613,386 10/1971 Klee 62/208 55 5'2 SPRA VALVE 56 g 57 CON TROL MO TORLN: SPRAY 62 HEADER THERMOCOUPLE (6A5 BALANCE) GAS BALANCE CONTROLDAMPER 6N2 HIGH VELOCITY FLOW L TE MP. CON TROL THERMO COUPLE PATENTEUmi 2 I975 SHEET PATENTEB AUG! 2M5 SHEET SHEET PAIENTEC PATENTEB AUG 121975 mu \muaq qm mqQ 1 CRYOGENIC REFRIGERATION APPARATUS WITH AUTOMATICTEMPERATURE CONTROL AND AUTOMATIC GAS BALANCE CONTROL Cross-Reference toRelated Applications This application is a continuation-in-part of myprior applications Ser. No. 264,133, filed June 19, 1972, now U.S. Pat.No. 3,824,806 dated July 23, 1974, and Ser. No. 390,755, flied Aug. 23,1973, now U.S. Pat. No. 3,871,186 dated Mar. 18, 1975.

BACKGROUND OF THE INVENTION One of the features of this invention is toprovide a refrigerating apparatus for refrigerating articles while theyare being transported through an insulated chamber or tunnel and whilecirculating refrigerant gas in the tunnel in contact with the articlestogether with temperature and gas control means in which the temperaturewithin the circulation loop is controlled by the flow of cryogenic fluidwhich provides more refrigerant to the tunnel when desired to maintain alow temperature and a gas balance control for proportioning gas flowbetween the circulation loop and a vent that is separate from the mainexhaust as a function of the temperature at the vent, all of which tendsto maintain optimum refrigeration temperature automatically within theinterior of the chamber or tunnel.

Another feature of the invention is to provide a method in which therefrigerating method comprises controlling the amount of refrigerantintroduced into the tunnel by the temperature in the circulation loop ofgas within the tunnel and selectively directing gas either toward theprincipal gas exhaust or toward a gas vent that are at adjacent ends ofthe chamber or tunnel by temperature of the gas at the vent.

The most pertinent prior art of which applicant is aware are U.S. Pat.Nos. 3,345,828, 3,403,527, 3,427,820, 3,431,745 and 3,600,901. None ofthese, however, disclose the features of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. la and 1b combine in a plan viewof an apparatus embodying the invention.

FIGS. 2a, 2b and 2c combine in a longitudinal sectional view through theapparatus of FIGS. 1a and lb.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the embodiment shown in thedrawings the apparatus for refrigerating articles comprises an elongatedfreezing tunnel 11 or chamber having top 12, bottom l3 and side 14 and15 insulated walls. The top insulated walls 12 are in the form of hingedlids 16 which may be raised and lowered by means of hydraulie cylinders17, in the customary manner, of which only two are illustrated in FIGS.la and lb and which are illustrated in side elevation in FIGS. 3, 4, 6and 7.

Positioned within the chamber or tunnel l 1 between an entrance l8 andan exit 19 thereto is a customary conveyor 20 for conveying articles inthe chamber in a path coinciding with the upper length 21 of theconveyor between the entrance and exit. One such article is illustratedschematically at 22 in FIG. 20.

The fluid cryogen is a liquid of which liquid nitrogen is an excellentexample for the illustrated embodiment. Liquid carbon dioxide is anotherexample. The liquid cryogen in either case generates a cryogen gaswithin the tunnel. The liquid nitrogen of this embodiment is introducedby means of a liquid spray system 23 which as illustrated is an assemblyof nozzles 24 arranged in the system 23 and spaced longitudinally andtransversely of the chamber.

Thus in the illustrated embodiment of FIG. 4 there are six nozzlesspanning the width of the chamber 11 and eight nozzles arrangedlongitudinally in each row as shown in FIG. 2a. The liquid cryogen issupplied to the nozzle assembly by means of a pipe 25 which communicateswith the piping 26 interconnecting the nozzles 24 by means of a flexiblehose 27. With this arrangement the entire liquid flow assembly 23 may beelevated for cleaning when the lids 16 are raised without requiringdismantling of the assembly 23.

In order to aid in the heat transfer the cryogen gas is circulated overthe articles within the chamber 1 I. This circulation aids the intimatecontact of the cryogen gas with the articles particularly where theconveyor 20 is of the preferred open mesh metal construction. Thecirculating means comprises a horizontal baffle 28 beneath the conveyor20 and thus beneath the articles 22 conveyed thereon with this baffle 28having an end 29 adjacent to but short of the entrance end 18 of thetunnel 1 1. The circulation flow which is caused by the rotatable blower30 is in a path illustrated by the circulation arrows 31 as shown inFIGS. 2b, 2c, 5, 6 and 7.

The circulation path as is illustrated most clearly in FIGS. 2b and 6includes a duct 32 located externally of the chamber 11. Thus as shownin FIG. 6 this duct is located beneath and to the side of the apparatus10 and communicates with the bottom of the chamber 11 by means of avertical fitting 33 that directs gas vertically downwardly into the duct32 and upwardly at the side through a passage 34 into the blower 30where the recirculated gas is introduced laterally back into the tunnelwhere it is deflected by curved spaced vanes 35 for longitudinal flowtoward the entrance 18 as illustrated in FIG. 5.

Beneath the spray head assembly 23 there is provided a pan 36 slopedtoward the circulation duct 32 to direct by gravity flow refrigerant gasand excess liquid from the spray zone 37 into the duct 32.

Communicating with the tunnel or chamber 11 adjacent the entrance 18 aregas outlet exhaust means 38 for exhausting 39 the cryogen gas from thecirculation loop 31. The baffle 28 which is adjacent the entrance 18 tothe tunnel and therefore adjacent the gas outlet exhaust means 38separates the chamber 1 1 into a first or upper reach 40 and a second orlower reach 41 of the loop 31. As illustrated in FIGS. 2b and 2c thereach 40 provides countercurrent direct heat exchange contact with thearticles 22. The second reach 41 gas is out of direct Contact with thearticles.

Provided in the chamber 11 above the conveyor 20 and in communicationwith the upper reach 40 are a plurality of gas moving devices 42 hereshown as vertical axis fans for providing vertical axial flowillustrated by the arrows 43. These fans 42 are longitudinally spacedfrom each other and in the illustrated embodiment are six in numberalthough more or less can be used depending upon the size of theapparatus.

As illustrated by the arrows 43 in FIG. 2b the exhaust from the fans isdownwardly through the conveyor and over the articles 22 carried thereonand is then directed back upwardly by impingement upon the baffle 28.The fans 42 serve as turbulizers to obtain better heat transfer betweenthe articles conveyed through the tunnel by the conveyor 20 and the gas.This results in the exhaust gases illustrated at 39 in FIG. 2c having ahigher temperature caused by greater heat extracted from the enteringarticles 22. Thus in one embodiment of the apparatus of this inventionthe temperature of the exhaust gases 39 was not more than 80 less thanthe temperature of the incoming articles 22. Furthermore, the transverseor in this embodiment vertical exhaust from the axial flow fans 44 is ina direction transverse to the longitudinal circulation flow path 31 withthe combination of gas flows at 90 to each other increasing theturbulence with corresponding increases in heat transfer so that ashorter tunnel l 1 may be employed for a given refrigerating capacity.

The apparatus is constructed in separable sections for cleaning. Such aseparable arrangement is disclosed in my prior US. Pat. No. 3,580,000.

Provided in the chamber or elongated tunnel 11 are a series oftriangular baffles 45 with the apex extending I downwardly as shown inFIG. and with the angle at the apex being about 90. These transversebaffles 45 which extend from side to side across the chamber serve asflow restriction means between the entrance end of the circulationbaffle 28 and the exhaust 38 for limiting the volumetric flow 46 of gasfrom the circulation loop 31 to the exhaust 38. This baffle limitationof the flow thereby serves to increase the volumetric flow in thecirculation loop 31 and thus its linear velocity, thereby insuringbetter heat transfer between the gas in the circulation loop and themoving series of articles 22 being refrigerated.

The top insulated wall 12 of the tunnel is raised in a portion 47 wherethe right angled turning vanes 35 serve to direct the circulating 31 gasback into the beginning of the upper or first reach 40 of thecirculation loop where the gas is directed as indicated by the arrow 31in FIG. 2c in countercurrent flow with respect to the direction ofmovement of the articles 22 by the conveyor 20. Located at this entrance48 to the circulation loop 31 there is provided a gas balance controlfor proportioning gas flow between the loop 31 and a gas vent 49 that isadjacent the tunnel exit 19.

In the illustrated embodiment this gas balance control comprises a firstdamper 50 that is hingedly mounted for swinging between a positionagainst the top wall 12 (FIG. 8) and a position at about 90 to theposition shown in FIG. 8 for proportioning the gas flow between thecirculation loop 31 and thus the exhaust 3 8 at the product entrance 18of the tunnel and the gas vent 49 at the product exit 19 of the tunnel.

This gas balance control damper 50 also serves to control the velocityof the gas flow in the circulation loop 31 that includes both the upperarticle contacting first reach '40. and the lower reach 41 as previouslydescribed with the two reaches being separated by the circulation baffle28. Theposition of this velocity and balance control damper 59 iscontrolled by a thermocouple 51 of the customary type operating a motorcontrol 52 also of the customary type such as illustrated by the motorcontrol 62in my prior US. Pat. No. 3,600,901. i

Gas flow from the gas vent 49 is at least sufficient to prevent ambientair from entering the tunnel during operation thereof under anyoperating conditions so as to prevent introducing warm air, moisture andthe like. The temperature within the tunnel 11 is controlled bycontrolling the rate of introduction of the cryogen as illustrated bythe liquid nitrogen sprays 53. Thus in the illustrated embodiment liquidnitrogen is supplied through a pipe 54 and a variable valve 55 to thespray header or assembly 23. Flow through this valve 55 is automaticallycontrolled by a control motor 56 whose operation is regulated by atemperature control instrument 57 of standard and well knownconstruction which in turn is controlled by a temperature controlthermocouple 58 that is positioned in the xternal duct portion 32 of thegas circulation loop. As is illustrated in FIG. 6 this thermocouple 58is preferably located in the side portion 34 of the duct 32 and adjacentthe entrance to the circulation fan or blower 30. Thus the temperaturecontrol means for the interior of the tunnel 11 functions in part bycontrolling the rate of introduction od cryogen as a function of gastemperature in the circulation loop 31 and in the embodiment disclosedspecifically by the temperature in the external portion 32 of this loop.

The interior of the tunnel 11 is also provided with a second damper 59that is located at the entrance 60 to the external duct portion 32 andat the adjacent end 61 of the baffle 28. The second damper 59 is movedbe tween nearly closed and completely open position by hand adjustment.

In operation the manual damper 59 is left slightly open as indicatedschematically in FIG. 8 and after start-up the balanced condition of aslight outward flow through vent 49 and exhaust flow through 38 ismaintained by the temperature controlled gas balance damper 50. If, dueto abnormal conditions within the tunnel, this is not sufficient tomaintain the desired balance, then the manual damper 59 is adjusted toprovide a wider space 60 so that more of the gas is drawn into therecirculation loop directly from .the spray zone thereby decreasing theflow out through the vent 49 and correspondingly increasing flow throughthe exhaust duct 38. No other adjustments are made on the manual damper59 unless conditions change so-as to require the re-adjustment. Typicaloperating conditions that would require such an adjustment of the manualdamper 59 are increased restrictions to flow in the recirculation path31 due to the bulk of the articles 22 being refrigerated or extremelylong tunnel structures which would themselves cause flow restrictions inthe recirculation path 31 or the like. At a balanced flow there is aslight purge of refrigerant gas outward through the vent 49 which isindicated by the desired temperature of from l0 to +lOF., or about 0F.,at the temperature sensitive element or thermocouple 51 so that there isno entrance of ambient air.

Once this condition has been achieved the manual damper 59 is untouchedand all adjustments necessary to allow for the various loads and'variousheat inputs are accomplished by lowering and raising the first damper 50in response to temperature fluctuations at the thermocouple 54 locatedat the gas vent 49. At minimum load when the passage of articles 22 isat a minimum or even temporarily suspended the damper 50 is in itslowered position which is about 90 from the elevated position of FIG. 8.

The liquid spray assembly 23 is located between the circulation baffle28 and the product exit 19 of the tunnel. The gas generated in the sprayzone flows both countercurrently to the articles as indicated by thearrows 62 of FIGS. and 2b and a portion into duct 32 where it enters thecirculation loop 31. The temperature within the chamber is thencontrolled bythe temperature in the circulation loop as sensed by thethermocouple 58 that controls the amount of liquid nitrogen fed to thespray head 23 by the pipe 62.

It has been customary in the past to exhaust gas from the tunnel as atthe illustrated exhaust duct 38 to accommodate the incoming fresh gas 62from the introduced refrigerant. Because the amount of gas that isvaporized from the fresh refrigerant is dependent upon the load ofarticles being passed through the tunnel and is thus variable the amountof gas withdrawn at the exhaust 38 must be adjusted to meet changingloads. However, it has been found difficult to regulate and balance therefrigerant and particularly the freezing operation by solely adjustingthe amount of exhaust gas as at the duct 38. This is true because eventhe minimum withdrawal of gas from the circulation path 31 at minimumloads is greater than the rate of gas generation and this results in airinflow at the exit end 19 of the tunnel.

With the apparatus and method of this invention there is always at leasta minimum amount of outward flow of gas at vent 49 and at 46 to prevententrance of ambient air in any quantity. Then the amount of freshrefrigerant introduced and thus the amount of gas generated iscontrolled for varying load conditions by the temperature sensor 58which as described controls the amount of fresh refrigerant introduced.Actually, the temperature controls as illustrated and described are sosensitive that under all operating conditions the temperature at rhevent 49 will be approximately 0F. and the temperature at the exhaustduct 38 will be about 80F. colder than the incoming product temperature.This condition is achieved not only by the variable damper 50 and itstemperature sensor 51 at the tunnel exit but also by recirculating thegas 31 through the tunnel and countercurrently to the articles passingtherethrough with this amount recirculated being controlled at least inpart by the previously described baffles 45.

Various parts of the apparatus shown in the drawings are similar tocorresponding parts of my earlier copending applications Ser. No.264,133, filed June 19,

:present application. In the first of these applications there isdisclosed an apparatus which uses liquid carbon dioxide as the cryogenand also a circulation baffle similar to the above baffle 28. Thecontrols and balances of this invention are equally applicable to aliquid carbon dioxide system as they are to a liquid nitrogen system.

The second of the above applications or Ser. No. 339,395 now US. Pat.No. 3,855,815 also discloses a system in which a flow restricting memberis provided between the gas circulation path and the gas exhaust toserve substantially the same exhaust control and circulation speed upeffects as the baffles 45 disclosed herein.

In the last of the above applications there is disclosed a systemsimilar to that disclosed herein but omitting the gas flow restrictorbaffles and the temperature and balance controls.

The gas balance control dampers and 59 and the temperature controls -58are in the prior art.

Having described my invention as related to the embodiment shown in theaccompanying drawings, it is my intention that the invention be notlimited by any of the details of description, unless otherwisespecified, but rather be construed broadly within its spirit and scopeas set out in the appended claims.

I claim:

1. Apparatus for refrigerating articles, comprising: means defining athermally isolated chamber having boundary walls and an entrance and anexit for passage of said articles through said chamber; conveyor meansin said chamber for said passage; introducing means for introducing acryogen means into said chamber for providing cryogen gas in saidchamber; circulating means for circulating said gas in a circulationloop including a first reach in direct heat exchange contact with saidarticles and a second reach out of said direct contact with saidarticles; gas outlet exhaust means from said chamber spaced from saidloop on the side thereof adjacent said chamber entrance; flowrestriction means in said chamber between said loop and said exhaust forlimiting volumetric gas flow from said loop to said exhaust and therebymaintaining a rapid gas flow rate in said loop; gas outlet vent meansfrom said chamber located adjacent to said chamber exit; temperatureresponsive control means for automatically controlling the rate of saidintroducing of cryogen as a function of gas temperature in said loop;and temperature responsive gas balance control means for automaticallyproportioning gas flow between said loop and said vent means as afunction of gas temperature at said vent means.

2. The apparatus of claim 1 wherein said temperature responsive controlmeans includes a temperature sensor sensing the temperature of said gasin said second reach.

3. The apparatus of claim 1 wherein there is provided a manuallyadjustable second damper between said introducing means and said secondreach.

4. The apparatus of claim 1 wherein said introducing means is locatedbetween said circulating means and said tunnel exit.

5. The apparatus of claim 1 wherein said temperature responsive gasbalance control means comprises a first damper means in said circulationloop movable from fully open position to loop intercepting position fordirecting gas selectively and proportionally from said loop toward saidvent means.

6. The apparatus of claim 5 wherein said loop has an area comprising acommon exit from the second reach and entrance to the first reach, andsaid first damper is located in said area.

7. The apparatus of claim wherein said loop has an area comprising acommon exit from the second reach and entrance to the first reach, saidfirst damper is located in said area, and there is provided a manuallyadjustable second damper between said introducing means and said secondreach.

8. The apparatus of claim 7 wherein said chamber comprises a tunnel,said walls comprise a top wall and a bottom wall and said first damperis positioned adjacent to said top wall and said second damper adjacentto said bottom wall.

9. The apparatus of claim 1 wherein said circulating means comprises abaffle in said chamber separating said first and second reaches of saidloop.

10. The apparatus of claim 9 wherein said baffle is longitudinal of andbetween the top and bottom of said chamber with the first reach and saidconveyor means being above the bafile and the second reach including thearea beneath the baffle.

11. The apparatus of claim 1 wherein said chamber comprises a tunnel,said second reach of said circulation loop includes a duct locatedexternally of the tunnel having a duct entrance for receiving gas fromsaid chamber and a duct exit for delivering gas to said first loopreach.

12. The apparatus of claim 11 wherein said temperature responsivecontrol means includes a temperature sensor sensing the temperature ofsaid gas in said second reach duct.

13. The apparatus of claim 11 wherein there is provided a manuallyadjustable second damper between said introducing means and said secondreach located at said'duct entrance.

14. The apparatus of claim 11 wherein said introducing means is locatedbetween said duct and said tunnel exit.

15. The apparatus of claim 1 wherein said chamber comprises an elongatedtunnel having a top wall and said means for restricting gas flowcomprises a baffle extending downwardly from the top wall and spacedabove the conveyor to permit said passage of the articles.

16. The apparatus of claim 15 wherein there is provided a plurality ofsaid baffles spaced along said top wall between said circulation loopand said gas outlet exhaust means.

1. Apparatus for refrigerating articles, comprising: means defining athermally isolated chamber having boundary walls and an entrance and anexit for passage of said articles through said chamber; conveyor meansin said chamber for said passage; introducing means for introducing acryogen means into said chamber for providing cryogen gas in saidchamber; circulating means for circulating said gas in a circulationloop including a first reach in direct heat exchange contact with saidarticles and a second reach out of said direct contact with saidarticles; gas outlet exhaust means from said chamber spaced from saidloop on the side thereof adjacent said chamber entrance; flowrestriction means in said chamber between said loop and said exhaust forlimiting volumetric gas flow from said loop to said exhaust and therebymaintaining a rapid gas flow rate in said loop; gas outlet vent meansfrom said chamber located adjacent to said chamber exit; temperatureresponsive control means for automatically controlling the rate of saidintroducing of cryogen as a function of gas temperature in said loop;and temperature responsive gas balance control means for automaticallyproportioning gas flow between said loop and said vent means as afunction of gas temperature at said vent means.
 2. The apparatus ofclaim 1 wherein said temperature responsive control means includes atemperature sensor sensing the temperature of said gas in said secondreach.
 3. The apparatus of claim 1 wherein there is provided a manuallyadjustable second damper between said introducing means and said secondreach.
 4. The apparatus of claim 1 wherein said introducing means islocated between said circulating means and said tunnel exit.
 5. Theapparatus of claim 1 wherein said temperature responsive gas balancecontrol means comprises a first damper means in said circulation loopmovable from fully open position to loop intercepting position fordirecting gas selectively and proportionally from said loop toward saidvent means.
 6. The apparatus of claim 5 wherein said loop has an areacomprising a common exit from the second reach and entrance to the firstreach, and said first damper is located in said area.
 7. The apparatusof claim 5 wherein said loop has an area comprising a common exit fromthe second reach and entrance to the first reach, said first damper islocated in said area, and there is provided a manually adjustable seconddamper between said introducing means and said second reach.
 8. Theapparatus of claim 7 wherein said chamber comprises a tunnel, said wallscomprise a top wall and a bottom wall and said first damper ispositioned adjacent to said top wall and said second damper adjacent tosaid bottom wall.
 9. The apparatus of claim 1 wherein said circulatingmeans comprises a baffle in said chamber separating said first andsecond reaches of said loop.
 10. The apparatus of claim 9 wherein saidbaffle is longitudinal of and between the top and bottom of said chamberwith the first reach and said conveyor means being above the baffle andthe second reach including the area beneath the baffle.
 11. Theapparatus of claim 1 wherein said chamber comprises a tunnel, saidsecond reach of said circulation loop includes a duct located externallyof the tunnel having a duct entrance for receiving gas from said chamberand a duct exit for delivering gas to said first loop reach.
 12. Theapparatus of claim 11 wherein said temperature responsive control meansincludes a temperature sensor sensing the temperature of said gas insaid second reach duct.
 13. The apparatus of claim 11 wherein there isprovided a manually adjustable second damper between said introducingmeans and said second reach located at said duct entrance.
 14. Theapparatus of clAim 11 wherein said introducing means is located betweensaid duct and said tunnel exit.
 15. The apparatus of claim 1 whereinsaid chamber comprises an elongated tunnel having a top wall and saidmeans for restricting gas flow comprises a baffle extending downwardlyfrom the top wall and spaced above the conveyor to permit said passageof the articles.
 16. The apparatus of claim 15 wherein there is provideda plurality of said baffles spaced along said top wall between saidcirculation loop and said gas outlet exhaust means.